In the field of front and rear wheel steering vehicles, it was previously proposed to use an electric actuator such as an electric servo motor to actuate the rear wheel steering device of the vehicle. In such a vehicle, the load or the resistance which the rear wheel steering device encounters as it is actuated becomes greater as the traveling speed of the vehicle decreases, and the steering load when the vehicle is stationary could be more than twice greater than that when the vehicle is at a cruising speed. Also, when the vehicle is rapidly steered on a road surface demonstrating a high frictional coefficient, the steering load could be substantially greater than that under normal condition.
The actuator for the rear wheel steering device is therefore required to have a sufficient capacity to meet the need under the highest possible load condition. However, such a high load condition is rarely encountered. In short, the conventional electric rear wheel steering device had to be equipped with a relatively expensive actuator to meet the need which seldom arises. Also, the size and weight of the rear wheel steering device were also objectionable.
It was proposed in the copending patent application Ser. Nos. 08/147,825 (now U.S. Pat. No. 5,521,820) to disable the integral control when the steering load has exceeded a certain threshold level so that the rear wheel steering device is controlled solely by the proportional and differential feedback control and the feed-forward control. According to this previous proposal, when the steering load is high, by disabling the integral control, some offset error will be created, but the actuator is prevented from overcompensating for the large accumulated error which would be otherwise created due to the lack of the power of the actuator. However, under a high steering load condition, the actual steering angle tends to be indeterminate, and this could cause the vehicle operator to feel the steering system unreliable. For instance, when the vehicle operator starts out the vehicle from a stationary condition while continually steering the rear wheels, the steering load of the rear wheel steering device which was initially high immediately falls below the threshold level as soon as the vehicle starts moving. In such situation, the steering angle of the rear wheels at the time when the vehicle actually starts moving tends to be indeterminate, and this could create an unfavorable impression of the handling of the vehicle.